A conventional image reading apparatus, in a prior art, which uses a contact type image-pickup unit called a CIS or the like, calibrates sensitivity variation (irregularity) of respective pixels of the image-pickup unit or irradiation intensity variation by an original document irradiation system, in other words, carries out shading calibration, based on image data obtained by reading a white reference member having a white colored surface serving as a reference.
In this situation, dust contamination of a white reference member on which shading data for shading calibration is based on provides incorrect shading data, which is unfavorable.
Accordingly, in order to prevent storing incorrect data, when there are pixel portions, which are lower than a predetermined level, in image data obtained as a result of reading of a white reference member, it is determined that colored dust is mixed in the portions, and shading calibration data is generated while performing a process to remove effects by the color dust. Herein, color dust generically means chromatic dust, and achromatic (gray to black) dust other than white color.
On the other hand, white dust, such as paper dust, cannot be distinguished from the white color of a white reference member, and may cause generation of incorrect shading calibration data.
In this situation, it is attempted to detect white dust when generating shading calibration data, such as disclosed in Patent Document 1.
Patent Document 1: Japanese Patent Application Publication TOKKAI No. 2001-86333 (page 1, FIG. 1)
In the invention disclosed by above Patent Document 1, the light intensity of a light source of an image reading device is switched so as to detect color dust with an ordinary light intensity, and detect white dust in a state where the illuminance on a white reference member is lowered by reducing the light intensity by half.
However, even when light intensity can be controlled, there is a problem that the control is not easy, or the light intensity cannot be controlled depending on the type of a lamp. Particularly, with xenon lamps which are used recently because of low heat generation, controlling of the light intensity is difficult, which does not allow application of control disclosed in Patent Document 1.
The present invention was devised to solve problems, as described above, and relates to dust detection which allows easy detection of white dust as well as color dust in an image reading apparatus that irradiates an original document with light and reads an image of the original document by photoelectrically converting reflected light from the original document with a contact type image pick-up unit.